T06: Amber REST client (httpx async)

Amber Electric REST API client using httpx async. Bearer auth via AMBER_API_TOKEN secret. 5s in-memory cache for current prices, 60s refresh cycle. Endpoints: GET /sites, /sites/{id}/prices/current, /prices/forecasts, /prices/next. Store buy+feed-in at 30-min granularity to prices table via sqlmodel. Error handling: 401 → cache last known + disable PRICE mode; timeout/retry 3x with exponential backoff.

Note (2026-06-29T02:16:50Z)

Picked up by ralph at 2026-06-29T14:05Z. Implementing Amber Energy async REST client for price/forecast data. Checking existing amber_client.py and tests.